(a) Technical Field
The present invention relates to an ejector for a fuel cell system, more particularly, to an ejector for supplying a fuel (such as hydrogen) to a stack in a fuel cell system.
(b) Description of the Related Art
In general, a fuel cell system is a kind of electric power generation system that converts chemical energy of a fuel into electric energy.
Such a fuel cell system includes a fuel cell stack, a fuel supply system for supplying a fuel (e.g., hydrogen) to the fuel cell stack, an air supply system for supplying oxygen in air, which is an oxidizing agent required for an electrochemical reaction, to the fuel cell stack, and a cooling system for exhausting reaction heat of the fuel cell stack to an outside of the system and controlling a driving temperature of the fuel cell stack.
As shown in FIG. 5 (RELATED ART), a fuel supply and recirculation system in a conventional fuel cell system basically includes a hydrogen supply line 110 connected to a hydrogen storage tank 100, a hydrogen recirculation line 120 through which the unreacted hydrogen of a fuel cell stack 170 is recirculated, an ejector 140a and 140b installed at a node where a stack inlet 130 and the hydrogen recirculation line 120 meet to pump new hydrogen and recirculated hydrogen toward an anode of a fuel cell stack, a stack inlet pressure sensor 150 installed on the hydrogen supply line 110 to measure the pressures of hydrogen and air, and a regulator 160 installed on the hydrogen supply line 110.
In this case, the ejector 140a and 140b ejects the compressed hydrogen supplied from a high-pressure tank through a nozzle to generate a vacuum, such that the exhaust gas in the fuel cell stack is absorbed to recirculation hydrogen gas.
A blower may be used as a recirculation means for the fuel cell system described above, but the blower, which is a motor-based actuator, is expensive and components such as bearings may be easily corroded due to condensate of the recirculation gas. In addition, when a rotational component is stuck due to the condensate, the rotational component may be thawed by use of a heater, which would undesirably increase the complexity of such an arrangement.
An ejector, which serves as a simple solution to the above-described problems, ejects a jet through a nozzle by using the hydrogen of about 100 barg at a rear end of a high-pressure regulator to generate a momentum that is required to supply and recirculate fuel at the same time.
However, when a diameter of the nozzle is enlarged, a speed of the jet decreases so that absorbing performance of the ejector deteriorates.
For this reason, although it is advantageous in performance to reduce a size of the ejector nozzle, there is a need to enlarge a throat area of the nozzle in order to supply a large amount of fluid for a great load.
As a possible solution, there has been proposed a technique of disposing a large ejector 140b and a small ejector 140a. 
As shown in FIG. 5, when a large ejector is provided, the ejector must use either a large nozzle or a small nozzle according to an amount of load such that the large nozzle is used for a large amount of load and the small nozzle is used for a small amount of load.
The performance of an ejector may be classified according to recirculation performance by fuel supply and pump. The larger the size of a nozzle may be, the larger amount of fuel flow may be supplied under the same pressure. However, when the nozzle is large, the pumping performance deteriorates due to a low jet speed for a small amount of fluid. Although a small size nozzle has good aerodynamic performance since the small size nozzle has a high flow speed even in low-level supply, the small size nozzle could not fulfill a request to supply a large amount of fluid.
In consideration of the above, an apparatus for controlling a supply of hydrogen fuel for a fuel cell system had been proposed in Korean Unexamined Patent Publication No. 10-2012-0136708.
As shown in FIG. 6 (RELATED ART), the apparatus for controlling the supply of hydrogen fuel for a fuel cell system includes an ejector 200 installed at a fuel cell stack inlet to supply hydrogen to the fuel cell stack and to form a recirculation flow, a proportional-control solenoid valve 220 connected to a hydrogen supply line to communicate with a nozzle inlet 210 of the ejector 200 to control the supply of hydrogen, and a valve controller (not shown) for controlling driving of the proportional-control solenoid valve 220 according to an output of the fuel cell system.
In this case, the valve controller controls the driving of the proportional-control solenoid valve in a scheme of pulse flow control in a low output section, where an output in a current state is less than a preset reference output.
Reference numeral 230 is an ejector nozzle, and reference numerals 240 and 250 represent an ejector outlet and a valve body, respectively.
According to the above technique, the performance of a low-load ejector is improved through a pulse hydrogen supply. The value controlling the pulse hydrogen supply, which is a proportional-control valve, supplies a pulse flow to the nozzle through a fast ON/OFF operation at a low load. In addition, under a great load condition as a fuel supply condition, quiet driving can be achieved through an up and down movement of a plunger.
However, in the above technique, a low load driving range must be set and the larger the range is, the greater the number of ON/OFF operations, so that a harsh request is generated in the valve.
In particular, it is required to perform the pulse flow through the least displacement in a small range if possible. It is the most ideal to obtain required aerodynamic performance without any pulse flow.
In other words, the pumping efficiency of an ejector is improved so that valve durability may be secured and noise generation may be suppressed.